The present invention is an oil well device used to control the flow of fluid within a well. More particularly, the present invention is a subsurface safety valve which opens and closes a tubing string to control the flow of oil or gas produced from the well. Properly installed, the apparatus is designed to shut in the well automatically in the event of abnormal high or low pressure fluctuations, explosion or malfunction of surface equipment, fire or other dangerous situations such as a sudden release of underground pressure of subsurface formation.
The tubing subsurface safety valve of the present invention contains an improved means for operating the valve member which is designed to permit a quick response to speed valve closing in the event of an accident. The valve actuator of this invention comprises a temperature responsive helical coil spring which is constructed of a material having a temperature actuated shape memory. The valve operation can be achieved by expanding the helical coil actuator upon application of electrical heat which is supplied from a downhole trasformer.
Oil and gas wells must be protected against certain potential hazards in the event of damage or failure of the surface equipment. For example, a sudden release of underground pressure may cause the well to go wild and out of control, resulting in an oil spill or possibly a fire. To prevent uncontrolled flow of fluids from the well caused by such an accident, it is common practice to install a downhole safety valve in the tubing through which the fluids are produced. Even under normal operating conditions it is often desirable to employ such a valve to interrupt flow at subsurface depths.
Many subsurface safety valves are surface controlled, utilizing a hydraulic control system. Fluid is pressurized or depressurized and pumped into the hydraulic control line to open and close the valve. Hydraulic pressure is maintained in the control line to hold the safety valve open, and in the event of accident, the pressure in the control line is released and the safety valve automatically closes. Although these hydraulically controlled safety valves are in widespread daily use, they are subject to certain disadvantages and shortcomings.
Among the serious shortcomings of certain of these hydraulically controlled valves is the fact that the apparatus has to be equipped with a bias spring, or other counterforce means to close the valve operating mechanism.
These bias means work during closing operation in opposition to the various control fluid forces which would resist and delay the closure of the valve. (For this reason, it has been found impractical to utilize such valves at a great depth in a well as a long delay between initiation of valve closure and complete closure may occur).
For a surface controlled subsurface safety valve having a single hydraulic control line, as disclosed in U.S. Pat. Nos. 3,375,874; 3,703,193; 4,086,935; 4,193,450 and 4,214,606; and U.K. Pat. No. 1,565,625, three control fluid forces resist valve closure. First, a hydrostatic pressure force, proportional to valve depth, is created due to the presence of control fluid within the control line. Second, a fluid frictional force is created due to the required displacement of a relatively large volume of control fluid from the safety valve into the small diameter control line during valve closure. Third, the inertia of the control fluid, which was initially at rest, and which must be displaced back into the control line also resists the valve closure.
Utilizing dual hydraulic control lines, as disclosed in U.S. Pat. No. 3,696,868 and as illustrated in the composite catalog No. 805 of safety systems by Production Equipment Division, Hydril Company, permits the first, hydrostatic pressure force to be counterbalanced and, in effect, nulified. However, valve closure is still resisted by fluid frictional forces and the inertia of the mass of control fluid at rest. Additionally, there are extra equipment costs and handling problems whenever a well installation incorporates dual control lines for a single subsurface safety valve.
In hydraulically controlled subsurface safety valves, means for moving the valve includes a pressure chamber. The pressure in the pressure chamber is normally enclosed by sealing elements as shown in U.S. Pat. Nos. 4,086,935 and 4,214,606. These seals are essentially used to isolate the pressure chamber from communicating with the tubing bore pressure zone. Thus, in the event of seal failure in the area of the control pressure chamber, fluid of production tubing may enter the control pressure chamber causing the valve to fail in the open position. If this occurs, the tubing string must be pulled out and the sealing elements must be replaced.
A further disadvantage of the hydraulically controlled subsurface safety valves is that the hydraulic control line leading to the subsurface valve is susceptible to be damaged, or corroded, or otherwise leaks to permit reduction of the hydraulic control pressure, the safety valve will close in accordance with its "failsafe design" and the well will be shut in. To restore production, the tubing string must be pulled and the hydraulic control line replaced.
Another disadvantage of the use of hydraulically controlled subsurface safety valves is that extremely high hydraulic control pressures are sometimes required. This means that hydraulic control lines at the surface must carry high hydraulic pressure. These high hydraulic control pressures constitute a potential hazard to personnel working on the platform and/or around the wellhead.
There are other examples of subsurface safety valves in the prior art which are controlled by means other than hydraulic pressure. These have involved the application of an electromagnetic operating mechanism controllable from the surface by electrical lead wires extending along the tubing annulus to the sealed solenoid coil at the downhole safety valve. By the transmission of electric current from the surface the valve is opened and held open. Interruption of the current supply results in automatic closure of the valve. Examples of this subsurface safety valve are shown in U.S. Pat. Nos. 4,161,215 and 4,191,248. Due to valve construction and operating characteristics there are numerous limitations confronting the designer that are difficult to resolve when using solenoids to operate a valve. These may result in a delayed response to valve closure during an accident. The use of solenoid operated valves has never been fully successful for various reasons and their use has been found to be impractical.
Therefore, it is an object of this invention to provide an improved subsurface well safety valve which overcomes the aforesaid problems.
It is a further object of this invention to provide a subsurface well safety valve which exhibits improved valve performance and operates without delayed valve closure even at great depths.
It is a further object of this invention to provide a subsurface well safety valve which elminates leakage of control system fluid and thus reduces valve malfunction.
It is a further object of this invention to provide an improved system for controlling operation of the aforesaid valve which offers enhanced safety for nearby personnel and may be operated from surface and subsurface locations of controllers.